Optical fiber cable and method of mid-span access thereof

ABSTRACT

An optical fiber cable is comprised of: a slotted core ( 7 ) elongated along an axis of the optical fiber cable, the slotted core including a slot ( 11 ) running in parallel with the axis and a groove ( 5 ) accessible through the slot; one or more optical fibers ( 3 ) placed in the groove; a sheath ( 9 ) enclosing the slotted core and the optical fibers; a bonding portion ( 15 ) where the slotted core is bonded with the sheath; and two or more strength members ( 17 ) embedded in the slotted core, the strength member running in parallel with the axis, and being aligned on a plane including the axis.

TECHNICAL FIELD

Apparatuses consistent with the present invention relate to opticalfiber cables enclosing fibers, in which enclosed fibers are easilyaccessible but prevented from damage, and a method of mid-span access ofthe optical fiber cables.

BACKGROUND ART

An optical fiber cable in some cases includes plural fibers for thepurpose of increasing the capacity or the number of devices linking viathe cable. These fibers may be enclosed with a slotted core and theslotted core along with the fibers may be further enclosed with asheath.

After being laid, some optical fiber cables are often subject to a worknamed “mid-span access” to make the enclosed optical fibers branch off.In the mid-span access work, the sheath and the core are cut and splitto enable access to one or more of the enclosed fibers. As the fibersexposed to the exterior are susceptible to damage, any proper measurefor protection may be required.

Japanese Patent Unexamined Application Publications Nos. S62-291608,S63-5313, H06-50009 and H08-211261 disclose related arts of opticalfiber cables.

DISCLOSURE OF INVENTION Technical Problem

Some circumstances cause damage to properties of the optical fibers. Forexample, as the slotted core is likely to move relative to the sheath,projection of the slotted core out of one end of the sheath may occur.The projection will lead to damage to the optical fibers at theprojecting part. Further, curving or meandering may generate compressionor tensile stress on the optical fibers, which causes increase intransmission loss. Certain embodiments of the present invention provideoptical fiber cables enclosing fibers, in which enclosed fibers areeasily accessible but prevented from damage.

Technical Solution

An optical fiber cable according to an aspect of the present inventionhas an axis. The optical fiber cable is comprised of: a slotted coreelongated along the axis, the slotted core including a slot running inparallel with the axis and a groove accessible through the slot; one ormore optical fibers placed in the groove; a sheath enclosing the slottedcore and the optical fibers; a bonding portion where the slotted core isbonded with the sheath; and two or more strength members embedded in theslotted core, the strength member running in parallel with the axis andbeing aligned on a plane including the axis.

According to another aspect of the present invention, a method ofmid-span access of the optical fiber cable is comprised of separating apart of a sheath from the optical fiber to expose the slot in part;covering the exposed slot in part with the separated part of the sheath;and pulling one or more of the optical fibers out of the slotted core.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of an optical fiber cable according to afirst embodiment of the present invention.

FIGS. 2(A) through 2(E) are drawings explaining a process of mid-spanaccess.

FIG. 3 is a schematic drawing explaining a draw test method.

FIG. 4 shows a cross section of an optical fiber cable according to asecond embodiment of the present invention.

FIG. 5 shows a cross section of an optical fiber cable according to afourth embodiment of the present invention.

FIG. 6 shows a cross section of an optical fiber cable according to afifth embodiment of the present invention.

FIG. 7 shows a cross section of an optical fiber cable according to asixth embodiment of the present invention.

FIG. 8 shows a cross section of an optical fiber cable according to aseventh embodiment of the present invention, which further applies toeighth and ninth embodiments of the present invention.

FIG. 9 shows a cross section of an optical fiber cable according to atenth embodiment of the present invention.

FIG. 10 shows a longitudinal section of the optical fiber taken alongthe Y-axis of FIG. 9.

FIG. 11 shows a cross section of an optical fiber cable according to aneleventh embodiment of the present invention.

FIGS. 12(A) through 12(C) are partial sectional views to show variationsof markers for indicating where a slotted core is fixed with a sheath.

FIG. 13 shows a cross section of an optical fiber according to anembodiment of the present invention, which is replaceable with that ofthe first embodiment.

FIG. 14 shows a cross section of an optical fiber according to afourteenth embodiment of the present invention.

FIGS. 15(A) through 15(D) are drawings explaining a process of mid-spanaccess explained in relation to the optical fiber cable according to thefourteenth embodiment.

FIG. 16 is a cross sectional view of the optical fiber cable forexplanation of the process of the mid-span access.

FIG. 17 is a cross sectional view of the optical fiber cable forillustrating a state after splitting.

BEST MODE FOR CARRYING OUT THE INVENTION

Exemplary embodiments of the present invention will be describedhereinafter with reference to the appended drawings. While optical fibercables according to the embodiments are elongated along a central axis Cthereof, FIGS. 1, 4-9, 11-13 only show cross sections thereof takenalong a plane perpendicular to the central axis. The followingdescriptions and the appended drawings often refer rectangularcoordinates represented by X- and Y-axes on these sectional planes forconvenience of explanation. These X- and Y-axes and elements relatedthereto sometimes represent planes and bodies elongated along thecentral axis C.

Referring to FIG. 1, an optical fiber cable 1 according to a firstembodiment of the present invention is comprised of optical fibers 3, aslotted core 7 having a groove 5 for housing the optical fibers 3, and asheath 9 enclosing the slotted core 9 along with the optical fibers 3.Needless to say, all the fibers 3, the groove 5, the core 7, the sheath9 and the slot 11 run in parallel with the central axis C of the opticalfiber cable 1.

The slotted core 7 is further comprised of a slot 11 opened linearlyalong the slotted core 7 for enabling access to the interior of thegroove 5. Therefore the slotted core 7 has a C-letter cross sectionalshape. The wall of the slotted core 7 gradually becomes thicker towardthe side opposite to the slot 11. The groove 5 is eccentric from theouter profile of the slotted core 7. When the center of the slot 11 andthe side just opposite to the slot 11 are made aligned on the Y-axis,the eccentricity is also in a direction along the Y-axis.

The sheath 9 preferably consists of any proper resin such aspolyethylene. The sheath 9 in comprised of a nonuniform wall whichgradually becomes thinner from a thickest wall portion 13 toward athinnest wall portion 15, both of which are aligned on the Y-axis.Thereby eccentricity in the direction along the Y-axis is given to ahollow defined by the wall relative to the outer profile of the sheath9. The thickest wall portion 13 covers the slot 11.

As the eccentricity of the groove 5 relative to the slotted core 7 isjust reverse to the eccentricity of the hollow of the sheath 9, thegroove 5 is resultantly substantially concentric with the central axis Cof the optical fiber cable 1. Alternatively, the groove 5 may beslightly eccentric from the central axis C in either direction along theY-axis.

The slotted core 7 is further comprised of a pair of strength members 17embedded therein. Both the strength members 17 are aligned on the planeincluding both the Y-axis and the central axis C of the cable 1.Further, the strength members 17 are in nature opposite to the slot 11with respect to the central axis C. The strength members 17 may beformed in various shapes such as a line, a strip, an elongatedmultilateral prism or a column. The number of the strength members 17 isnot limited to two and may be three or more. A slight interval ispreferably held between the strength members 17.

The strength members 17 are made of any material reinforcing the opticalfiber cable 1 against tensile force, such as steel or FRP (FiberReinforced Plastic), and in general have greater stiffness than those ofthe other members. As the strength members 17 having such stiffness arealigned on the plane, when the optical fiber cable 1 is curved, thisplane functions as a neutral surface in a meaning of mechanics (asurface along which material is neither compressed nor extended).

In any case, the strength members 17 may be aligned on another plane.Even then, if the optical fibers 3 are disposed around the plane,increase in transmission loss can be suppressed as will be discussedlater.

Although the cross sectional shape of the groove 5 is illustrated as acircle in FIG. 1, the shape is not limited thereto and may be an ellipseor any irregular shape instead. Further, the interior of the groove 5may be either vacant except the optical fibers 3 or filled with anybuffer members. In any case, the optical fibers 3 are preferablydisposed around the central axis C.

The optical fibers 3 may be any of bare optical fibers, optical fibercords, and optical fiber ribbons.

An elongate tape 19 preferably made of non-woven fabric or any resinsuch as PET (PolyEthylene Terephthalate) is attached on the slotted core7 to cover the slot 11. The elongate tape 19 is not wrapped around theslotted core 7 and leaves a lower part of a surface of the slotted core7 uncovered. Therefore the sheath 9 may be directly in contact with thislower part of the slotted core 7 while the elongate tape 19 intervenesbetween the upper part of the slotted core 7 and the sheath 9.

At this uncovered part, the slotted core 7 has a bonding portion 23where the slotted core 7 is bonded to the sheath 9. The bonding portion23 longitudinally ranges over the slotted core 7 to form a continuousline or a row of separate portions at intervals. Thermal fusion bodingmay be applied to bonding at the bonding portion 23. In the presentembodiment, a projecting rib 21 projecting from the slotted core 7 isformed in advance of bonding. The projecting rib 21 facilitates thermalfusion bonding with the sheath 9 and, after bonding, becomes the bondingportion 23 fitting in and bonding with a complementary recess of thesheath 9. In any proper case, thermal fusion bonding or any otherbonding treatment can be omitted and the projection rib 21 fitting inthe recess by itself functions as bond. Preferably the projecting rib 21does not project out of the sheath 9.

The optical fiber cable 1 may include a rip cord to facilitate splittingthe sheath 9.

As already discussed, the plane on which the strength members 17 arealigned, shown as the Y-axis in FIG. 1, functions as a neutral surfacein a meaning of mechanics when the optical fiber cable 1 is curved inany directions perpendicular to the plane (namely, in the direction ofthe X-axis). Moreover the optical fiber cable 1 can substantially freelyrotate or twist with very small stress, because the cable 1 is ingeneral subject to very small restriction. Thus, even if one would curvethe optical fiber cable 1 in a direction deviated from the X-axis, theoptical fiber cable 1 would be slightly reoriented to have itself curvedin the X-axis and then the plane including the central axis C stillfunctions as a neutral surface. Further, as the optical fibers 3 aredisposed around the central axis C (included in the neutral surface),the optical fibers 3 are substantially neither compressed nor extended.Therefore transmission loss caused by compression or tensile stress canbe suppressed in a very low level. It is advantageous in view ofsuppression of transmission loss particularly when some circumstancesforce a laid optical fiber cable to curve or meander.

As the sheath 9 is free from a strength member which may be obstructiveto splitting or cutting out the sheath 9, the optical fiber cable 1provides facility for the mid-span access work.

As the sheath 9 has a nonuniform wall in which the thickest wall portion13 covers the slot 11, mechanical strength in this part is reinforced.This is advantageous in view of prevention of damage to the enclosedoptical fibers 3 when external force is applied to the sheath 9, inparticular over the slot 11. This effect becomes remarkable when thethickness of the thickest wall portion 13 is 1.5 times or more of thethickness of the thinnest wall portion 15.

Without the bonding portion 23, the slotted core 7 is likely to move inits longitudinal direction because temperature change after laying theoptical fiber cable 1 may cause thermal expansion or contraction.Further, some manners of handling of the optical fiber cable 1 may causerotational displacement of the slotted core 7 relative to the sheath 9.As the sheath 9 and the slotted core 7 are bonded together at thebonding portion 23, the slotted core 7 is prevented from displacementrelative to the sheath 9 in both the longitudinal and rotationaldirections. The bond at the bonding portion 23 effectively preventsprojection, retraction and rotational displacement of the slotted core7. As the bond at the bonding portion 23 prevents such displacement, theoptical fiber cable 1 provides prominent facility for handling.

The bond between the slotted core 7 and the sheath 9 is limited in thebonding portion 23. This fact provides facility for the mid-span accesswork because peeling of the sheath 9 is easily carried out as comparedwith a case where the core and the sheath are entirely bonded together.In particular, while a cutter is put into the sheath at the beginning ofthe mid-span access work, the cutter may cut out the projecting rib 21and therefore simultaneously break the bond between the slotted core 7and the sheath 9 at the bonding portion 23. Thus workability about themid-span access work is prominently improved.

Referring to FIGS. 2(A) through 2(E), a process of mid-span access willbe described hereinafter. First, a sharp edge of a cutter 25 is put intothe sheath 9 and made advance around the circumference thereof to carryout cutting around of the sheath 9. Further the cutter 25 is madeadvance along the bonding portion 23 to carry out splitting of thesheath 9 and subsequently cutting around is carried out again at anotherpart as shown in FIG. 2(B). As the bonding portion 23 is formed at theprojecting rib 21, the bond between the slotted core 7 and the sheath 9is easily broken in the course of the aforementioned step. Further,while movement of the cutter 25 along the longitudinal direction givesrise to damage to the enclosed optical fibers 3 as the slot 11 runs inthis direction, such movement is made at the bonding portion 23 oppositeto the slot 11 and therefore the optical fibers 3 are not subject todamage.

Then a part of the sheath 9 is separated from the remainder of thesheath 9 and further a longitudinal slit 27 is formed in this part asshown in FIG. 2(C). This part becomes removable from the slotted core 7and the optical fibers 3.

After removal of the part of the sheath 9, the slot 11 is exposed asshown in FIG. 2(D) and therefore the optical fibers 3 housed in thegroove 5 become accessible through the slot 11. One or more of theoptical fibers 3 are pulled out of the slotted core 7 as shown in FIG.2(E) and then subject to a branching process.

Table 1 demonstrates test results of some examples in regard to adrawing test, projection length of the slotted core at the end of thesheath, workability about the mid-span access work, and transmissionloss. The drawing test had been carried out in a manner shown in FIG. 3,in which a slotted core 7 of a test piece 29 is drawn from a sheath 9having a length of 400 mm in a speed of 100 mm/min in a directionindicated by an arrow therein and a maximum value of force of drawing ismeasured.

Meanwhile, the force of drawing is preferably 98N or more in view ofprevention of displacement of the slotted core relative to the sheath.

The working example 1 is produced in accordance with the presentembodiment. Comparative examples 1-5 are different from the presentembodiment in structural parameters as summarized in this table.

TABLE 1 MAIN WORKING COMPARATIVE COMPARATIVE COMPARATIVE COMPARATIVECOMPARATIVE FEATURES EXAMPLE 1 EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4EXAMPLE 5 STRUC- FIXATION PARTIAL OVERALL NONE PRESS ON NONE NONE TURALBETWEEN THE ADHESION THE SLOT BY PARAM- SLOT CORE THE SHEATH ETERS ANDTHE SHEATH TAPE OR A TAPE ALONG A TAPE ALONG A TAPE ALONG A TAPE ALONG ASPIRAL A TAPE TOTALLY WRAPPING THE SLOT IN THE SLOT IN THE SLOT THE SLOTWRAPPING ALONG THE PART, PART, IN PART, IN PART, AROUND SLOT, AND A NOWRAPPING NO WRAPPING NO WRAPPING NO WRAPPING THE SLOT YARN WOUNDTHEREAROUND EVALU- FORCE OF 98 N OR MORE 98 N OR MORE 10 N OR LESS 98 NOR MORE 85 N 20 N ATION DRAWING THE SLOT CORE (FIG. 3) (SHEATH LENGTH:40 cm) A PROJECTION NONE NONE A PROJECTION A PROJECTION A PROJECTION APROJECTION FROM THE (1 mm OR LESS) (1 mm OR OF 55 mm OF 5 mm OF ABOUT 5mm OF 36 mm END OF LESS) IN LENGTH IN LENGTH IN LENGTH IN LENGTH THESHEATH WORKABILITY EXCELLENT POOR EXCELLENT TOLERABLE TOLERABLETOLERABLE *2 (EXTRA WORK (EXTRA WORK FOR REMOVAL) FOR REMOVAL)TRANSMISSION 0.21 dB/km 0.21 dB/km 0.21 dB/km 0.45 dB/km 0.23 dB/km 0.21dB/km LOSS (AT THE WAVELENGTH OF 1.55 μm) *NOTES EXCELLENT: WORKABLEWITHOUT ANY PROBLEMS AND WITH GREATER FACILITY THAN EXISTING CABLESTOLERABLE: WORKABLE WITH CONSIDERABLE LABOR AS COMPARED WITH THAT FOREXISTING CABLES POOR: TROUBLESOME IN WORK

As being understood from Table 1, the working example 1 in accordancewith the present embodiment has satisfactory properties in that theforce of drawing is 98N or more, the projection length is 1 mm or less,and the transmission loss is only 0.21 dB/km while workability about themid-span access work is excellent.

The comparative example 1 is different from the working example 1 inthat the slotted core 7 and the sheath 9 are totally bonded together.Workability about the mid-span access work is inferior to that of theworking example 1 because it is considerably laborious to peel off thesheath 9 totally bonded with the slotted core 7.

The comparative example 2 is different from the working example 1 inthat no bond is formed between the slotted core and the sheath. Thisstructure results in relatively small force of 10N or less required todraw the slotted core out of the sheath and a relatively largeprojection length of 55 mm of the slotted core out of the sheath. Thismeans that the slotted core is susceptible to displacement relative tothe sheath.

The comparative example 3 is different from the working example 1 inthat fixation of the slotted core with the sheath depends only onpressure of the sheath onto the slotted core. This structure results ina relatively large projection length of 5 mm of the slotted core out ofthe sheath. Further, workability about the mid-span access work isinferior to that of the working example 1. Transmission loss increasesup to 0.45 dB/km which is considerably larger than 0.21 dB/km of theworking example 1.

The comparative example 4 is different from the working example 1 inthat no bond is formed between the slotted core and the sheath and awrapping made of a tape is wound around the slotted core in a spiralshape. As the wrapping serves for friction against displacement of theslotted core, force of drawing is relatively high, 85N. However,projection length of the slotted core out of the sheath reaches about 5mm. Further, workability about the mid-span access work is inferior tothat of the working example 1 as extra work to remove the wrapping isrequired. Transmission is relatively low, 0.23 dB/km, although thisvalue is slightly larger than that of the working example 1.

The comparative example 4 is different from the working example 1 inthat no bond is formed between the slotted core and the sheath andfurther a wrapping made of a yarn is wound around the slotted core alongwith the elongate tape along the slot. While the wrapping serves forfriction against displacement of the slotted core, force of drawing theslotted core is only 20N and projection length of the slotted core outof the sheath reaches 36 mm. Further, workability about the mid-spanaccess work is inferior to that of the working example 1 as extra workto remove the wrapping is required. Transmission loss is fairly low,0.21 dB/km.

As being understood from the aforementioned comparisons, the workingexample 1 in accordance with the present embodiment provides beneficialresults as compared with the comparative examples, such as prevention ofdisplacement of the slotted core relative to the sheath, lowtransmission loss, and excellent workability about the mid-span accesswork.

The aforementioned embodiment will be modified in various ways. Some ofsuch modifications will be exemplarily described hereinafter. In thefollowing descriptions, differences compared with the aforementionedembodiment will be mainly described and descriptions about elementssubstantially identical to those of the aforementioned embodiment willbe omitted or simplified.

Referring to FIG. 4 which illustrates a second embodiment, the slottedcore 7 is in part given roughness in advance of bonding and the roughsurface of the slotted core 7 is subject to thermal fusion bonding toform a bonding portion 23 with the sheath 9. The bonding portion 23 iscomposed of a thermal fusion bonding portion 31 produced by the thermalfusion bonding, where the slotted core 7 and the sheath 9 are fusedtogether and thereby locally form a unitary body.

Alternatively, in a third embodiment, the slotted core 7 is in partheated up to a temperature sufficiently close to, or higher than, thatof the sheath 9 in advance of bonding so as to cause softening of theslotted core 7, and then thermal fusion bonding is carried out.

Referring to FIG. 5 which illustrates a fourth embodiment, instead ofthermal fusion bonding, a bonding material 33 such as adhesive may beused to form bond between the slotted core 7 and the sheath 9. Thereforethe bonding portion 23 is composed of the bonding material 33.

Referring to FIG. 6 which illustrates a fifth embodiment, instead of aprojecting rib, a recess 35 receding into the slotted core 7 is formedin advance of bonding and the sheath 9 has a complementary projection.After fitting the projection in the recess 35, thermal fusion bonding iscarried out to form a bonding portion 23 therebetween. As with the ribof the first embodiment, the recess 35 may be either a continuous lineof a concavity or a row of separate concavities, which longitudinallyranges over the slotted core 7.

Referring to FIG. 7 which illustrates a sixth embodiment, a rip cord 37having a bonding material such as adhesive is interposed between theslotted core 7 and the sheath 9. The rip cord 37 is preferably disposedon or close to the plane on which the strength members 17 are aligned.By means of the bonding material of the rip cord 37 instead of thermalfusion bonding, the slotted core 7 is bonded with the sheath 9. When therip cord 37 is drawn, it helps to split the sheath as an ordinary ripcord does, and further simultaneously breaks bond between the slottedcore 7 and the sheath 9. As in this way removal of the sheath 9 isfurther facilitated, one can further easily execute mid-span access workas compared with the first embodiment.

Referring to FIG. 8 which illustrates a seventh embodiment, one or moreabsorptive yarns 39 may be housed in the groove 5 of the slotted core 7.The absorptive yarns 39 improve quality of being waterproof of theoptical fiber cable 1.

Alternatively, an absorptive tape 41 having capacity to absorb water maybe applied instead of, or along with, the elongate tape 19 of the firstembodiment. The absorptive tape 41 also improves quality of beingwaterproof.

Further alternatively, both the absorptive yarns 39 and the absorptivetape 41 may be used. This combination of the absorptive yarns 39 and theabsorptive tape 41 further improves quality of being waterproof.

Referring to FIGS. 9 and 10 which illustrate a tenth embodiment, anchors43 are provided in the groove 5 of the slotted core 7. The anchors 43support one or more of the optical fibers 3 in place. Preferably, theanchors 43 are disposed at intervals in a direction along the centralaxis C. This structure prevents undesirable force acting on the opticalfibers 3 even when the optical fiber cable 1 is deformed. Moreover, theanchors 43 are preferably made of any soft viscous material. Preferablythe material is a UV-setting resin having a Young's modulus of 800 MPaor less and a viscosity of 500 cps or more at the normal temperature,whereby preventing undesirable force acting on the optical fibers 3,which may increase transmission loss. Preferably, the intervals of therespective anchors 43 are in a range from 100 mm to 2000 mm, wherebypreventing undesirable force acting on the optical fibers 3. Preferably,the support of the optical fibers 3 by the anchors 43 is regulated sothat a force required to draw out the supported optical fibers are 5N/10m or more.

Installation of the anchors 43 is executed in, but not limited to, thefollowing way. The tape 19 is uncoiled and then made to run. An uncuredUV-setting resin is intermittently injected onto the running tape 19substantially at the center thereof. Then the tape 19 along with theuncured UV-setting resin is exposed to UV light so as to cure theUV-setting resin and is subsequently turned around upside down. Therebythe anchors 43 made of the UV-setting resin are disposed at intervals onthe lower face of the tape 19. On the other hand, the optical fibers 3are put in the groove 5 of the slotted core 7 and the slot 11 is madeoriented upward. The tape 19 along with the anchors 43 is attached onthe slotted core 7 so as to cover the slot 11, whereby the anchors 43are inserted in the groove 5 to support the optical fibers 3. Anextruder may be used to enclose the slotted core 7 with the sheath 9.

Table 2 demonstrates test results of some examples in regard totransmission loss, a drawing test and workability about the mid-spanaccess. The examples 1-8 are in general manufactured in accordance withthe aforementioned tenth embodiment and vary in kinds of resin, Young'smoduli thereof, and viscosities thereof, as summarized in Table 2.

TABLE 2 TARGET EXAM- EXAM- EXAM- EXAM- EXAM- EXAM- VALUE PLE 1 PLE 2 PLE3 PLE 4 PLE 5 PLE 6 EXAMPLE 7 EXAMPLE 8 CONDI- FIXATION OF THE — BY A BYA BY A BY A BY A UV BY A UV BY A BY A FILLED TION OPTICAL FIBER UV UV UVUV RESIN RESIN HOT-MELT YARN RESIN RESIN RESIN RESIN ADHESIVE YOUNG'SMODULUS — 500 1000 500 800 1000 600 — 1000 OF THE RESIN (Mpa) VISCOSITYOF — 300 300 500 500 500 700 GREATER 500 THE RESIN (cps) THAN 10000 RE-TRANSMISSION LESS 0.22 0.28 0.21 0.23 0.32 0.20 0.32 0.86 SULTS LOSS(dB/km) THAN 0.25 FORCE OF GREAT- 2.8 4.2 9.8 8.5 12 11.5 4 11 DRAWINGTHE ER SLOT CORE THAN 5 (N/10 m) WORKABILITY — EX- EX- EX- EX- EXCEL-EXCEL- TOLERABLE TOLERABLE CEL- CEL- CEL- CEL- LENT LENT (EXTRA WORK(EXTRA WORK LENT LENT LENT LENT FOR REMOVAL FOR REMOVAL OF THE OF THEYARN) HOT-MELT ADHESION)

Provided that a target level of performance is set such that atransmission loss is 0.25 dB/km or less, a force required to draw outthe slotted core from the sheath is greater than 5, and workabilityabout the mid-span access work is beyond that of existing cables, whatmeet the target level among the examples are the example 3, 4 and 6,which are commonly comprised of anchors made of the UV-setting resin.Both the example 7 having anchors made of hot-melt adhesive and theexample 8 in which yarns filled in the groove fix the optical fibers donot meet the target level.

In more detail, the examples 2, 5 and 8 do not have sufficiently lowtransmission loss which meets the target level as the Young's moduli ofthe anchors of these examples reach 1000 MPa. In contrast, the examples1, 3, 4 and 6 meet the target transmission loss, in which the Young'smoduli of the anchors are 800 MPa or less. More specifically, anchor'sYoung's moduli of 800 MPa or less provide beneficial results in view ofsuppression of transmission loss.

Further, the examples 1 and 2 in which the viscosities of the anchorsare 300 cps do not meet the target force of drawing, whereas theexamples 3, 4, 5 and 6 in which the viscosities are 500 cps or more meetthe target force of drawing. More specifically, anchor's viscosities of500 cps or more provide beneficial results in view of prevention ofdisplacement of the slotted core.

Further modification of the above embodiments will occur. Referring toFIG. 11 which illustrate an eleventh embodiment, widths of the slot 11in a proper range also beneficial results. A plane emanating from thecentral axis C in contact with an edge in the right of the slot 11 isshown as a line L in FIG. 11 and another plane emanating from thecentral axis C in contact with another edge in the left of the slot 11is shown as a line L′. These planes make an angle “theta” as shown inFIG. 11. When the angle theta is larger than 30 degrees, workabilityabout the mid-span access work becomes easy. Further, when the angletheta is less than 90 degrees, the sheath 9 is prevented from fallinginto the groove 5 and therefore does not have undesirable influence ontransmission loss. More specifically, the angles theta in a range from30 degrees to 90 degrees provide beneficial results.

Further, widths of the tape 19 in a proper range also beneficialresults. A plane emanating from the central axis C in contact with anedge in the right of the tape 19 is shown as a line T in FIG. 11 andanother plane emanating from the central axis C in contact with anotheredge in the left of the tape 19 is shown as a line T′. These planes makean angle “gamma” as shown in FIG. 11. Beneficial results provided byangles gamma larger than the angle theta would be needless to say. Whenthe angle gamma is less than four times the angle theta, the slottedcore 7 is securely fixed with the sheath 9 as the slotted core 7 and thesheath 9 ensure sufficient contact area. More specifically, the anglesgamma in a range from the angle gamma to four times the angle gammaprovide beneficial results.

Referring to FIGS. 12(A)-12(C) which illustrate a twelfth embodiment,the optical fiber cable 1 may be further comprised of a marker forindicating a position of the bonding portion 23. The maker may be aprojection 45 projecting from the sheath 9, which is just aligned to thebonding portion 23 as shown in FIG. 12(A). Alternatively, the marker maybe a colored bar 47 on the sheath 9 as shown in FIG. 12(B). Furtheralternatively, the marker may be a concave portion 49 as shown in FIG.12(C). Existence of the marker helps one who would carry out themid-span access work to find out where to cut.

Referring to FIG. 13 which illustrates a thirteenth embodiment, a pairof rectangular-prism-shape strength members 20 are embedded in theslotted core 7, instead of the columnar strength members 17 of the firstembodiment.

Referring to FIG. 14 which illustrates a fourteenth embodiment, theoptical fiber cable 1 is comprised of two notches 51 formed on an outersurface of the sheath 9, which run along the central axis C. The notches51 facilitate splitting of the sheath 9 at the time of the mid-spanaccess process. The number of the notches 51 is not limited to two butmay be three or more. Further, the sections of the notches 51 are notlimited to a V-letter shape but may be any proper shape. The otheraspects of the current embodiment may be identical to those of the firstembodiment or the other.

The notches 51 (or, outermost two of the notches 51 in a case wherethree or more notches are provided) may be disposed symmetrically withrespect to the thinnest wall portion 15 and leave a properly narrowinterval therebetween. Thus positioned notches 51 are located atrelatively thin portions of the sheath 9 and relatively close to eachother, thereby sufficiently facilitating splitting of the sheath 9. Apart of the sheath 9 separated from the remaining sheath 9 may bebeneficially reused for the purpose of covering the exposed slot 11 aswill be described later.

The breadth between the notches 51 affects easiness of re-enclosure ofthe optical fibers and easiness of ripping of the sheath. The testresults summarized in Table 3 demonstrate this feature. In these tests,test pieces 1-6 respectively having various breadth were tested. InTable 3, breadths are indicated by angles alpha, where an angle alpha isdefined as an angle NCN′ formed by planes CN, CN′ emanating from thecentral axis C and respectively passing the notches 51.

TABLE 3 TEST TEST TEST TEST TEST TEST PIECE 1 PIECE 2 PIECE 3 PIECE 4PIECE 5 PIECE 6 CONDITION BREADTH 15° 20° 30° 90° 160° 180° BETWEENNOTCHES (ANGLE α) RESULTS EASINESS OF DIFFICULT NOT SO EASY EASY EASYEASY DIFFICULT RE-ENCLOSURE DIFFICULT TO REQUIRE EASY TO THE FIBERS OFOPTICAL COVER STRESS TO RE-ENCLOSE ARE LIKELY FIBERS THE C-GROUP OPENAND THE FIBERS TO RUN OFF CORES WITH CLOSE THE ONCE THE SHEATH SEPARATEDSHEATH EASINESS OF DIFFICULT DIFFICULT EASY EASY EASY EASY RIPPING OFUNEXPECTED UNEXPECTED SPLIT AT THE THE SHEATH SPLIT MAY SPLIT MAYNOTCHES AS OCCUR OCCUR EXPECTED

As will be understood from Table 3, two or more notches on a sheathfacilitate splitting of the sheath, much more do those havingalpha-angles greater than 30 degrees (test pieces 3-6). Meanwhile, thetest pieces 3-5 having alpha-angles in a range in a range from 30degrees to 160 degrees are more advantageous in light of easiness ofre-enclosure of optical fibers than the test pieces 1, 2 and 6 havingalpha-angles out of the range.

The depth t of the notches 51 may be set in a particular range inrelation to the thickness D of the sheath 9 at the part of the notches51. Relatively small depths t in a range satisfying an inequality oft<D/5 are disadvantageous in light of easiness of splitting, and toolarge depths t in a range satisfying an inequality of t>4D/5 aredisadvantageous in light of toughness of the sheath 9. Therefore thedepth t in a range from D/5 to 4D/5 may be preferable.

Referring to FIGS. 15 and 16, a process of mid-span access will bedescribed in relation to the optical fiber cable according to the fourthembodiment.

Referring to FIG. 15(A), a sharp edge of a cutter 25 is put into thesheath 9 and made advance around the circumference thereof to carry outcutting around of the sheath 9. Movement of a sharp end of the cutter 25may be on a track illustrated as a round broken line in FIG. 16.

Next, as shown in FIG. 15(B) and FIG. 17, the sheath 9 is made splitwith the help of the notches 51. Then the sheath 9 is partly separatedinto a larger part and a smaller part, thereby the slotted core 7 ispartly exposed in between these parts.

Referring to FIG. 15C, the smaller part of the sheath 9 is allowed to becut out but the larger part is preferably left. One or more of theoptical fibers 3 are pulled out of the slotted core 7 and then subjectto a branching process. The rest of the optical fibers 3 are returned toand re-enclosed with the slotted core 7.

Then the uncut part of the sheath 9 can be reused for the purpose ofcovering the exposed slot 11 as shown in FIG. 15D. While the opticalfiber(s) 3 branching off the optical fiber cable 1 is led out of thesheath 9, the remaining optical fibers 3 are re-enclosed with the sheath9. This process does not require any specific jigs and can be executedby hands and fingers.

The fourteenth embodiment provides such reusability of the split sheath9 as a cover of the exposed optical fibers 3, while such reuse may bemade possible even in the other embodiments when some additional laborsare added. In contrast, in the prior arts, an exposed core requires anytape wrapped around the core or such protection means so as to avoiddamage to the optical fibers. Wrapping a tape on the core isconsiderably laborious. Therefore, the fourteenth embodiment providesboth excellent protection and easy workability.

The aforementioned first through fourteenth embodiments are compatiblewith each other. Therefore, any combination of these embodiments willoccur. Further, additional rip cords may be interposed between theslotted core 7 and the sheath 9.

Although the invention has been described above by reference to certainexemplary embodiments of the invention, the invention is not limited tothe exemplary embodiments described above. Modifications and variationsof the embodiments described above will occur to those skilled in theart, in light of the above teachings.

INDUSTRIAL APPLICABILITY

Optical fiber cables enclosing fibers, in which enclosed fibers areeasily accessible but prevented from damage, are provided.

1. An optical fiber cable having an axis, the optical fiber comprising:a slotted core elongated along the axis, the slotted core including aslot running in parallel with the axis and a groove accessible throughthe slot; one or more optical fibers placed in the groove; a sheathenclosing the slotted core and the optical fibers; a bonding portionwhere the slotted core is bonded with the sheath; and two or morestrength members embedded in the slotted core, the strength memberrunning in parallel with the axis and being aligned on a plane includingthe axis.
 2. The optical fiber cable of claim 1, wherein the strengthmembers include one selected from the group of steel and FRP.
 3. Theoptical fiber cable of claim 1, wherein the bonding portion includes oneselected from the group consisting of a projecting rib projecting fromthe slotted core, a binding element interposed between the slotted coreand the sheath, a recess receding in the slotted core, and a stringhaving adhesive interposed between the slotted core and the sheath. 4.The optical fiber cable of claim 1, further comprising: an absorptiveyarn placed in the groove.
 5. The optical fiber cable of claim 1,further comprising: an elongate tape attached on the slotted core tocover the slot.
 6. The optical fiber cable of claim 5, wherein theelongate tape includes an absorptive tape.
 7. The optical fiber cable ofclaim 5, wherein the bonding portion is left uncovered by the elongatetape and aligned with the slot and the strength members on the plane. 8.The optical fiber cable of claim 5, wherein the slot and the elongatetape are so dimensioned that a theta-angle formed by planes emanatingfrom the axis and respectively in contact with edges of the slot of theslotted core is in a range from 30 degrees to 90 degrees, and agamma-angle formed by another planes emanating from the axis andrespectively in contact with both edges of the elongate tape is greaterthan the theta-angle and smaller than four times the theta-angle.
 9. Theoptical fiber cable of claim 1, further comprising: one or more anchorsconfigured to support one or more of the optical fibers, the anchorsbeing disposed at intervals in a direction along the axis.
 10. Theoptical fiber cable of claim 9, wherein each of the anchors includes aUV-setting resin having a Young's modulus of 800 MPa or less and aviscosity of 500 cps or more at a normal temperature, each of theintervals between the anchors is in a range from 100 mm to 2000 mm, anda force required to draw out the supported optical fiber is 5N/10 m ormore.
 11. The optical fiber cable of claim 1, wherein the sheathincludes a nonuniform wall so that a largest thickness of the wall is1.5 times or more of a smallest thickness of the wall.
 12. The opticalfiber cable of claim 1, further comprising: a marker formed on thesheath, the marker indicating a position of the bonding portion.
 13. Theoptical fiber cable of claim 1, further comprising: two or more notchesformed on an outer surface of the sheath and running along the axis. 14.The optical fiber cable of claim 13, wherein the notched are disposed soas to have an alpha-angle formed by planes emanating from the axis andrespectively passing two of the notches in a range from 30 degrees to160 degrees.
 15. A method of mid-span access of the optical fiber cableof claim 1, the method comprising: splitting the sheath in part toexpose the slot in part; pulling one or more of the optical fibers outof the slotted core; and covering the exposed slot in part with a splitpart of the sheath.